This is an old revision of the document!
Starting with Sparky 7.0 “Orion Belt”, the default configuration of GRUB bootloader does not “see” other operating systems as default.
To fix that do:
If the grub config file does not exist, copy it, as root:
sudo cp /etc/default/grub.ucf-dist /etc/dafault/grub
Then edit it, as root, using any text editor, for example nano:
sudo nano /etc/default/grub
adding the following line:
GRUB_DISABLE_OS_PROBER=false
on the end of the file.
Then update grub:
sudo update-initramfs -u -k all sudo update-grub